Metal oxide semiconductors are generally suitable for large area and high resolution. Such metal oxide semiconductors have been used in an organic electro luminescence (EL) display and liquid crystal display (LCD). Conventionally, four mask processes have been employed to manufacture a thin film transistor using a metal oxide semiconductor. In order to perform the four mask processes, a back channel etching (BCE) method needs to be applied.
However, in the conventional method for manufacturing a thin film transistor according to the back channel etching method, there has been a problem of a partial loss of a surface of the metal oxide semiconductor. Specifically, during an etching process, an etchant may cause a partial loss of a surface of the metal oxide semiconductor. The loss of the surface of the metal oxide semiconductor results in degradation of electrical characteristics of the thin film transistor and thus needs to be solved.